mirror of
https://github.com/adulau/aha.git
synced 2024-12-30 20:56:23 +00:00
670e9f34ee
Remove many duplicated words under Documentation/ and do other small
cleanups.
Examples:
"and and" --> "and"
"in in" --> "in"
"the the" --> "the"
"the the" --> "to the"
...
Signed-off-by: Paolo Ornati <ornati@fastwebnet.it>
Signed-off-by: Adrian Bunk <bunk@stusta.de>
123 lines
4.1 KiB
Text
123 lines
4.1 KiB
Text
File management in the Linux kernel
|
|
-----------------------------------
|
|
|
|
This document describes how locking for files (struct file)
|
|
and file descriptor table (struct files) works.
|
|
|
|
Up until 2.6.12, the file descriptor table has been protected
|
|
with a lock (files->file_lock) and reference count (files->count).
|
|
->file_lock protected accesses to all the file related fields
|
|
of the table. ->count was used for sharing the file descriptor
|
|
table between tasks cloned with CLONE_FILES flag. Typically
|
|
this would be the case for posix threads. As with the common
|
|
refcounting model in the kernel, the last task doing
|
|
a put_files_struct() frees the file descriptor (fd) table.
|
|
The files (struct file) themselves are protected using
|
|
reference count (->f_count).
|
|
|
|
In the new lock-free model of file descriptor management,
|
|
the reference counting is similar, but the locking is
|
|
based on RCU. The file descriptor table contains multiple
|
|
elements - the fd sets (open_fds and close_on_exec, the
|
|
array of file pointers, the sizes of the sets and the array
|
|
etc.). In order for the updates to appear atomic to
|
|
a lock-free reader, all the elements of the file descriptor
|
|
table are in a separate structure - struct fdtable.
|
|
files_struct contains a pointer to struct fdtable through
|
|
which the actual fd table is accessed. Initially the
|
|
fdtable is embedded in files_struct itself. On a subsequent
|
|
expansion of fdtable, a new fdtable structure is allocated
|
|
and files->fdtab points to the new structure. The fdtable
|
|
structure is freed with RCU and lock-free readers either
|
|
see the old fdtable or the new fdtable making the update
|
|
appear atomic. Here are the locking rules for
|
|
the fdtable structure -
|
|
|
|
1. All references to the fdtable must be done through
|
|
the files_fdtable() macro :
|
|
|
|
struct fdtable *fdt;
|
|
|
|
rcu_read_lock();
|
|
|
|
fdt = files_fdtable(files);
|
|
....
|
|
if (n <= fdt->max_fds)
|
|
....
|
|
...
|
|
rcu_read_unlock();
|
|
|
|
files_fdtable() uses rcu_dereference() macro which takes care of
|
|
the memory barrier requirements for lock-free dereference.
|
|
The fdtable pointer must be read within the read-side
|
|
critical section.
|
|
|
|
2. Reading of the fdtable as described above must be protected
|
|
by rcu_read_lock()/rcu_read_unlock().
|
|
|
|
3. For any update to the fd table, files->file_lock must
|
|
be held.
|
|
|
|
4. To look up the file structure given an fd, a reader
|
|
must use either fcheck() or fcheck_files() APIs. These
|
|
take care of barrier requirements due to lock-free lookup.
|
|
An example :
|
|
|
|
struct file *file;
|
|
|
|
rcu_read_lock();
|
|
file = fcheck(fd);
|
|
if (file) {
|
|
...
|
|
}
|
|
....
|
|
rcu_read_unlock();
|
|
|
|
5. Handling of the file structures is special. Since the look-up
|
|
of the fd (fget()/fget_light()) are lock-free, it is possible
|
|
that look-up may race with the last put() operation on the
|
|
file structure. This is avoided using the rcuref APIs
|
|
on ->f_count :
|
|
|
|
rcu_read_lock();
|
|
file = fcheck_files(files, fd);
|
|
if (file) {
|
|
if (rcuref_inc_lf(&file->f_count))
|
|
*fput_needed = 1;
|
|
else
|
|
/* Didn't get the reference, someone's freed */
|
|
file = NULL;
|
|
}
|
|
rcu_read_unlock();
|
|
....
|
|
return file;
|
|
|
|
rcuref_inc_lf() detects if refcounts is already zero or
|
|
goes to zero during increment. If it does, we fail
|
|
fget()/fget_light().
|
|
|
|
6. Since both fdtable and file structures can be looked up
|
|
lock-free, they must be installed using rcu_assign_pointer()
|
|
API. If they are looked up lock-free, rcu_dereference()
|
|
must be used. However it is advisable to use files_fdtable()
|
|
and fcheck()/fcheck_files() which take care of these issues.
|
|
|
|
7. While updating, the fdtable pointer must be looked up while
|
|
holding files->file_lock. If ->file_lock is dropped, then
|
|
another thread expand the files thereby creating a new
|
|
fdtable and making the earlier fdtable pointer stale.
|
|
For example :
|
|
|
|
spin_lock(&files->file_lock);
|
|
fd = locate_fd(files, file, start);
|
|
if (fd >= 0) {
|
|
/* locate_fd() may have expanded fdtable, load the ptr */
|
|
fdt = files_fdtable(files);
|
|
FD_SET(fd, fdt->open_fds);
|
|
FD_CLR(fd, fdt->close_on_exec);
|
|
spin_unlock(&files->file_lock);
|
|
.....
|
|
|
|
Since locate_fd() can drop ->file_lock (and reacquire ->file_lock),
|
|
the fdtable pointer (fdt) must be loaded after locate_fd().
|
|
|